No standardized susceptibility testing procedures exist for isolates of rapidly growing mycobacteria or Nocardia species. Several different procedures have been suggested including broth microdilution and the E-test (manufactured by AB Biodisk, Culver City, CA). We are comparing both a microdilution procedure (JustOne, manufactured by Radiometer America, Inc., Westlake, OH) and the E- test for their reproducibility, looking simultaneously at a number of other variables such as inoculum concentration, incubation time, and testing medium. The JustOne strips are useful because they allow selection of dilutions of individual antimicrobial agents that may be particularly relevant clinically, rather than the use of the whole array of agents usually found in the microdilution plate format. We hope to select a procedure that will provide rapid and reproducible results and not be so technically demanding that it is unsuitable for the routine diagnostic laboratory.